The present invention relates to a spot joining method of metal members and a spot joining apparatus of metal members, and more specifically to a spot joining method and a spot joining apparatus in which different kinds of metal members are joined by a frictional joining method.
Recently, some vehicle bodies made of light metals, such as aluminum, have been manufactured aiming at lightweight vehicles. Light metals such as aluminum, however, have somewhat difficulties in joining by an electric resistance welding. Accordingly, there were problems that their joining methods might be limited and manufacturing costs might increase.
Meanwhile, a frictional agitation joining apparatus disclosed in Japanese Patent Laid-Open Publication No. 2003-117668 has been proposed. In this frictional agitation joining apparatus, a rotator having a pin at its tip is rotated at a high speed and then pressed against a work (an aluminum plate). The work is softened and agitated thereby, and a spot joining can be applied to the work.
Herein, manufacturing costs such as coolant water, welding current can be reduced properly compared with conventional spot resistance welding. The above-described patent publication also discloses a changing of a rotational speed and a pressing force, aiming at useful facilities for a mass production.
Meanwhile, although the vehicle body being made of aluminum members or the like is effective in manufacturing lightweight vehicles, steel members or the like may be used at some portions which need higher rigidity from viewpoints of strength or costs. In this case, different kinds of metal members, such as an aluminum member and a steel member, need to be joined to each other to manufacture the vehicle body.
Herein, although the above-described frictional agitation joining apparatus may be available for joining different kinds of metal members, it is not easy to agitate, especially, the aluminum member and the steel member properly, which have different melting points from each other. Accordingly, some countermeasures for this has been needed.
Herein, another type of joining method may be adopted, in which only the aluminum member is agitated by use of friction, and both new uncovered surfaces of the steel and aluminum members are joined via a so-called solid state joining. This solid state joining is a method of joining metal members in solid state without melting materials of the metal members, in which atomic diffusions occur around joining portions and the metal members are joined to each other with atoms of members being mixed with each other. Here, since an oxidation film is generally created on a surface of the steel member, a sufficient joining strength may not be obtained properly without any useful measures.
The applicant previously filed a U.S. patent application (Ser. No. 10/863,216) as to a method and the like of improving joining strength of such a solid state joining by eliminating the inappropriate oxidation film properly. This patent application has proposed the method comprising the steps of preventing the oxidation film from existing on the surface of the steel member by previously forming a zinc plating layer on the surface of steel member, pushing away the zinc plating layer out of joining portions by the frictional agitation of a rotating tool, and contacting both new uncovered surfaces of the members directly, thereby improving joining strength of the solid state joining. According to this method, the joining strength of solid state joining of different kinds of metal members can be improved properly.
However, there may be another difficulty in adopting this proposed method. Namely, the pin provided at the rotating tool is configured so as to be short enough to agitate only the aluminum member properly without reaching the steel member. In this case, however, such a short pin may provide somewhat difficulty in positioning the rotating pin at a specified portion stably, resulting in an unstable joining position. This unstable joining position may cause an improper joining. As a result, there may occur other problems that aluminum materials are accumulated on the rotating tool, an appearance of joining surfaces deteriorates, the sufficient joining strength is not obtained, and the like.